1. Field of the Invention
The present invention relates to spectrometric measurement apparatuses, image evaluation apparatuses, and image forming apparatuses.
2. Description of the Related Art
In recent years, in the field of production printing, sheet-feed printing press machines and printers for continuous slip/form paper have been digitalized. Accordingly, numerous products using the electrophotographic method and the inkjet method have been introduced. Furthermore, user needs have increased for multidimensional, high-precision, and high-density images, with the trend switching from monochrome printing to color printing. Accordingly, services delivered to consumers have become versatile. For example, there have been various advertisements catering to personal needs such as high quality photograph printing, catalog printing, and account printing. Thus, there is increased demand for high quality images, confirmation of personal information, and color reproduction.
Various techniques have been introduced in the market for increasing the image quality of image forming apparatuses. One example is providing an electrophotographic type image forming apparatus with a density sensor for detecting the toner density of unfixed toner on an intermediate transfer body or a photoconductive body, to stabilize the amount of toner supply. Another example is for confirming personal information. Specifically, regardless of the image forming method, the output image is photographed with a camera, and the personal information is detected by variance detection based on character recognition or a difference image between plural images. Yet another example is for color reproduction. Specifically, a color patch is output, and colors are measured at one or more points with a spectrometer to execute calibration.
The above techniques are preferably performed across the entire image area, in order to accommodate image variations between different pages or within the same page.
Patent documents 1 through 5 propose evaluation methods executed in the process of measuring properties across the full width of the image.
Patent document 1 discloses a technique of arranging plural line-type light receiving elements, and providing a mechanism for moving a measurement target relative to a detection system, to measure the spectral properties across the full width of the image. A light shielding wall is provided so that reflection light crosstalk does not occur at the detection target area between the light receiving elements.
Patent document 2 discloses a technique of continuously radiating the full width of the image with light sources having different wavelength bands, and acquiring the reflection light to acquire spectral properties across the full width of the image.
Patent document 3 discloses a technique of radiating light onto the full width of a print side, detecting the density in a particular region with a line sensor camera, and averaging the density, to compare the reference densities.
Patent document 4 discloses a technique of scanning an original document and a special original document several times, and determining the similarity between the two documents by obtaining the logical sum between the images with regard to common color information.
Patent document 5 discloses a technique of radiating light onto the full width of a print side, and acquiring spectral properties across the full width of the image with the use of a combination of a CCD (Charge Coupled Device) having a two-dimensional pixel structure and a diffraction element or a refraction element.
As described above, typical configurations for measuring colors of an image across the full width of the image include the following examples. One example is irradiating the image with plural light beams having different wavelength bands and moving the measuring system and the detection target with respect to each other while photographing the image with an area sensor or a line sensor. Another example is providing plural photographing systems, and limiting the wavelength band of the reflection light from the detection target that enters the photographing system. If a positional shift occurs in the detection target while acquiring plural wavelength bands of images, the color information at the respective detection target points cannot be precisely measured. Examples of methods of precisely measuring color information in plural images having different wavelength bands are as follows. One example is comparing the intensity of the reflection light acquired at the detection target position in an image with that of an original image or original data serving as a reference. Another example is applying the Wiener estimation technique based on the intensity of reflection light acquired from the detection target positions in the images, to estimate continuous distribution properties. Thus, if the detection target positions are different in the images, errors may arise in the process of making comparisons with a reference or in the process of estimating continuous distribution properties.
Patent document 1 discloses a line-type measuring system for measuring the colors across the full width of the image that is the detection target. However, this technique does not reduce positional shifts in the images obtained at the respective wavelength bands.
Patent document 2 discloses a technique of continuously irradiating the image with light sources having different wavelength bands, and acquiring the reflection light from the detection target. However, with this configuration, a shift may occur in the time axis, and therefore it is not possible to measure the same position in the detection target. Even if plural combinations of light sources and light receiving systems are provided, the detection target positions are highly likely to shift in images of different wavelength bands. Furthermore, patent document 2 discloses a configuration of using plural rows of detectors that are filtered by different colors. In this case also, positional shifts in the images may occur among the plural colors.
Patent document 3 discloses a technique of averaging the density in the detection region, and the average value is assumed to be the representative value. However, the value obtained as the distribution of the detection target is unreliable.
Patent document 4 discloses a technique of determining the similarity between two documents by comparing an original document with a detection target by inter-image calculation for each wavelength band. However, with this method, the color variation of the detection target cannot be specified. Furthermore, in a case where an image is reconstituted based on color information items of images that are separately obtained, it cannot be determined as to whether the color variations have occurred in the actual detection target.
Patent document 5 discloses a technique of using a CCD having a two-dimensional pixel structure. The image data is acquired in one direction, and spectroscopic data is acquired in the other direction, in order to specify the color information across the full width of the image. However, in a CCD having a two-dimensional pixel structure, the reading speed is considerably slower than that of the line sensor due to limitations in the data reading properties. Accordingly, it takes a long time to acquire color information of the detection target.
As described above, in the conventional technology, with a spectroscopic sensor for measuring spectroscopic information across the full width of the target image, it has been difficult to read data at high speed while acquiring high-precision spectroscopic data.    Patent Document 1: Japanese National Publication (Japanese translation of PCT application) No. 2008-518218    Patent Document 2: Japanese Laid-Open Patent Application No. 2005-315883    Patent Document 3: Japanese Laid-Open Patent Application No. 2002-310799    Patent Document 4: Japanese Patent No. 3566334    Patent Document 5: Japanese Laid-Open Patent Application No. 2003-139702